DEAR MAYO CLINIC: Our newborn was diagnosed with a problem in one of his kidneys called hydronephrosis, and we were told he eventually may need surgery. What caused this? How will the condition affect him in the future?

ANSWER: Hydronephrosis is a condition in which urine stays in the kidney instead of flowing out of it. That causes the kidney to swell. A range of medical conditions can lead to hydronephrosis. Treatment often depends on the underlying cause. Although surgery is sometimes needed, in many cases hydronephrosis resolves on its own as a child grows, and surgery is not necessary.

Essentially, hydronephrosis can be boiled down to a problem of urine flow. Normally, urine goes from the kidney to the tube that drains the kidney, called the ureter, to the bladder and then out of the body. In certain cases, however, urine backs up or remains inside the kidney or in the ureter. It’s in these cases that hydronephrosis develops.

One common reason for hydronephrosis is a partial blockage in the urinary tract. These blockages often form where the kidney meets the ureter, at a point called the ureteropelvic junction. Less commonly, they may occur where the ureter meets the bladder at what's called the ureterovesical junction.

Another frequent cause of hydronephrosis is a condition called vesicoureteral reflux. This happens when urine flows backward through the ureter from the bladder up into the kidney. Normally, the ureter is a one-way street. Urine flowing the wrong way makes it difficult for the kidney to empty properly and causes the kidney to swell.

Other less common causes of hydronephrosis in children include kidney stones, a tumor in the abdomen or pelvis, and problems with nerves that lead to the bladder.

When hydronephrosis is found in a fetus during pregnancy, an evaluation of the kidneys and the bladder often can be done using imaging exams while the baby is still in utero. This gives doctors a better idea of how the condition may need to be managed after birth.

When the baby is born, more tests are done to identify the source of the problem. The tests usually consist of an ultrasound of the kidneys and bladder, along with another test called a voiding cystourethrogram. These tests of the urinary tract allow doctors to examine the kidneys, bladder and ureters carefully. Depending on the results from these two tests, further imaging may be necessary.

In cases of mild and sometimes even moderate hydronephrosis without reflux, surgery typically is not needed. The hydronephrosis tends to improve on its own over time. If this is the case for your child, his doctor may recommend antibiotics to lower the risk of urinary tract infections.

When hydronephrosis makes it hard for the kidney to function — as can happen in more severe cases or in cases that involve reflux — surgery may be recommended to eliminate the blockage or correct the reflux.

If left untreated, hydronephrosis eventually can lead to permanent kidney damage. Rarely, it can cause kidney failure. In most cases, however, this condition does resolve successfully. In the meantime, because hydronephrosis typically affects only one kidney, the other kidney can do the work for both.

As you consider your options going forward, it’s best to do so in consultation with a board-certified pediatric urologist. He or she can help you decide the tests needed to determine the source of the hydronephrosis and, based on that, the best approach to treatment for your child. — Dr. Patricio Gargollo, Pediactric Urologist, Mayo Clinic Children's Center, Rochester, Minnesota.